1. Field of the Invention
The present invention relates generally to fluid seals for gas turbine engines. In particular, the invention relates to a seal having low windage loss properties. The seal includes a multi-celled seal land with a corrugated strip height H. A corrugated facing strip with a reduced corrugation height h is attached to exposed end faces of the seal land to simplify construction of the seal land and to provide a low windage loss surface at exposed seal land end faces.
2. Description of the Known Art
Gas turbine engines typically include seals to restrict the flow of the gases between rotating and non-rotating components. Control of leakage through such seals is important because excessive gas leakage degrades engine performance. For instance, a compressor discharge pressure seal is used to restrict the flow of compressor discharge air between rotating compressor components and stationary combustor components. Tight seal clearances are critical, because air leaking past the seal is lost to the engine power cycle. Air that passes through the seal is typically used for cooling downstream engine components.
The tight clearances required for such seals are difficult to maintain because of relative motion of the rotating and non-rotating engine components during engine operation. Vibration, aircraft maneuver loads, and differential thermal growth contribute to this relative movement. Labyrinth type seals are commonly used to accommodate the relative movement of the engine components. Labyrinth seals allow rubbing to occur between rotating and non-rotating components without damage to the seal function or the engine components. Such a labyrinth type seal is shown in U.S. Pat. No. 4,554,789 assigned to the assignee of the present invention. A similar sealing arrangement for accommodating clearances between rotating turbine blades and a stationary case is shown in U.S. Pat. No. 2,963,307 assigned to the assignee of the present invention.
Labyrinth seals include circumferential teeth mounted on a first engine component that face an abradable sealing surface, or seal land, mounted on a second engine component that is rotatable with respect to the first engine component. The teeth cut into the abradable seal land during relative motions between the rotating and non-rotating engine components. As a result, relative motion of the engine components can be accommodated without deforming the seal labyrinth teeth or damaging the engine.
The abradable seal land is commonly a honeycomb structure made of corrugated semi-hexagonal strips forming multiple hexagonal cells. Such hexagonal honeycomb structures are shown in U.S. Pat. No. 4,346,904 issued to Watkins, and U.S. Pat. No. 4,395,196 issued to Plautz.
U.S. Pat. No. 3,916,054 to Long et al. and U.S. Reissue Pat. No. Re. 30,600, a reissue of U.S. Pat. No. 4,022,481 to Long et al., describe a compliant structure comprising an assembly of corrugated strips, where the strips are fixed relative to each other on at least one side of the assembly, and describe combinations of corrugation configurations. U.S. Pat. No. 4,618,152 to Campbell discloses a honeycomb seal structure, and a method of fabricating such a structure.
Honeycomb seal lands are used because less heat is generated during rubbing between the rotating and non rotating engine components. The heat generated during a rub is a function of the amount of material the seal teeth rub against. Honeycomb seal lands are less dense than solid seal lands, and therefore generate less heat during engine rubs.
Such corrugated seal lands typically present a rough, irregular surface to the seal air flow at the seal land sides, or end faces, due to the corrugated strips that are exposed to the seal air flow at the seal land end faces. Because there is aerodynamic drag on the air passing through the seal between the rotating and non-rotating components, there is heating of the air as it passes through the seal. Cooling air passing through the seal is heated by friction induced work done on the cooling air as the air passes over surfaces on the seal, such as the corrugated strips at the seal land end faces. The amount of heating of the air is dependent on the exposed surface area and on the surface roughness. Surface areas and roughness of both the rotating and non-rotating components are important in this respect. Therefore, if the surface roughness can be reduced on part of the exposed surface area of either rotating or non-rotating seal component, the heating of the air passing through the seal can be reduced. Heating of cooling air by friction induced work is also commonly known as windage loss, and is detrimental to engine performance.
Gas turbine engine efficiency is increased by raising turbine operating temperatures. However, as operating temperatures are increased, the thermal limits of certain engine components may be exceeded. Cooling air is required to maintain the temperatures of these components within acceptable limits. Windage losses increase the temperature of the cooling air and reduce the cooling capability of the cooling air. U.S. Pat. No. 4,554,789 listed above addresses heating of cooling air by friction induced work at the labyrinth seals, but does not address heating of the cooling air caused by seal land corrugations.
Prior designs have attempted to reduce windage losses at the seal land end faces by attaching flat strip segments to the seal land sides, or end faces, thereby covering the exposed corrugated strips. Such flat strips require time consuming and costly manual fitup and rework, since they are not adapted to machine automated seal land production. Further, experience has shown the flat strips tend to buckle or warp, and break away from the seal land during braze operations and during engine operation due to differential thermal growth effects.